Method for forming composite articles

ABSTRACT

A composite structure is formed by depositing a one or more coatings on an open-cell foam skeleton to form a higher-density composite foam. In accordance with one aspect of the invention, the composite foam can be a carbon/carbon composite formed by a rapid densification process. The composite structure is suitable for use, for example, as a friction material employed in clutch and brake devices.

RELATED APPLICATIONS

[0001] This application is a divisional of U.S. application Ser. No.09/857,949, filed Dec. 17, 1999, with a Deposit Date of Aug. 8, 2001,which is the U.S. National stage of International Application No.PCT/US99/30140, filed Dec. 17, 1999, published in English, which claimsthe benefit of U.S. Provisional Application No. 60/112,704, filed Dec.18, 1998. The entire teachings of the above applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] Relatively-dense carbon/carbon composites have been found to behighly useful in a variety of structural applications. Because ofcertain characteristics, such as high strength, high stiffness, lightweight, high temperature resistance, and advantageous frictionalproperties, these composites are desirably suited for use, for example,in the aerospace and automotive brake pad industries. They are favoredfor use in high-end automotive transmissions but high costs haveprevented widespread utilization.

[0003] Dense carbon/carbon composite structures typically include acarbon fiber matrix, wherein the interstices in the fiber matrix are atleast partially filled with deposited carbon. The carbon fibers are highin strength, and are typically in the form of a woven or non-wovenfabric or mat. In either case, the carbon fibers provide the compositewith structural reinforcement.

[0004] To fill the carbon fabric with additional carbon, the fibers aretypically placed in a chamber, where they are heated and exposed to acarbon-based vapor. Carbon from the vapor is thereby deposited on theheated fabric via chemical vapor deposition.

[0005] In an alternative method for depositing carbon, the fiber fabricis placed in a chamber filled with liquid precursor (cyclohexane, forexample), and the fibers are heated to pyrolize the liquid precursor atthe surface of the fabric. The pyrolysis of the precursor produces avapor that deposits carbon on the fibers within the fabric. This processis referred to as “rapid densification” and is described in greaterdetail in U.S. Pat. No. 5,389,152, issued to Thurston et al.

[0006] Though the above-described methods are known to producehigh-quality composite structures, the commercial application of thesestructures is limited by the high cost of carbon fibers, processing andenergy consumption. Accordingly, the application of these methods tomass-production industries, such as automobile manufacturing, has thusfar been greatly limited due to economic feasibility. Further, due tothe axially-elongated structure of the fibers, the composite propertiesare generally non-isotropic and highly dependent on fiber orientation.

SUMMARY OF THE INVENTION

[0007] The invention is generally directed to a method of forming acomposite foam, the composite foam, and articles, such as clutch andbrake components, formed of the composite foam.

[0008] In methods of this invention, a composite foam is formed bydepositing one or several layers of a coating on an open-cell foamreticulated skeleton. The coatings may be metallic, ceramic,carbonaceous etc.

[0009] In accordance with one aspect of a method of the invention, thereticulated foam skeleton is contacted with a liquid precursor. Thereticulated foam skeleton is heated to pyrolize the liquid precursor andcause a product of the pyrolized liquid precursor to deposit on thereticulated foam skeleton, thereby forming a composite foam of a higherdensity than the starting material. The sequence of material types thatconstitute the various layers may be varied.

[0010] In accordance with another aspect of a method of the invention, areticulated carbon skeleton is formed by pyrolizing a polymeric foam.Carbon then is deposited on the carbon skeleton to form a carbon/carboncomposite foam with a solid density of greater than 30%.

[0011] A carbon foam of this invention, which can be formed by theabove-described methods, includes an open lattice of carbon ligamentsthat form a network of three-dimensionally interconnected cells and apyrolytic carbon coating on the open lattice. The solid density of thecarbon foam is greater than 30%.

[0012] Articles formed of the carbon foam include a clutch or brakedevice with a pair of members mounted for relative rotation andengagement. The carbon foam serves as a friction material that isrotatable with the members and includes confronting surfaces.

[0013] The methods and apparatus of this invention provide numerousadvantages. For example, the cost of carbon foam is generally lower thanthat of carbon fibers. Therefore, methods of this invention cansignificantly reduce the cost of forming substantially-densecarbon/carbon composite foams. Accordingly, a broader range ofapplications can now be economically justified for use of carbon/carboncomposites. Further, the resulting foams can have a substantiallyisotropic and openly-porous structure relative to that of many materialsthat employ fibers. Because the composite is substantially isotropic,performance of a frictional surface comprising a foam of this inventionis likely to be more uniform and consistent as the friction surfacewears. The ceramic composite foams also have a relatively highpermeability, thereby facilitating hydraulic flow in many wet-frictionapplications. Moreover, distortion from machining is reduced due to theisotropic structure, thereby promoting flatness and parallelism inmachined friction surfaces. The methods of this invention also can beused to produce exceptionally dense structures with over 50% soliddensity, while retaining open porosity.

[0014] Though a carbon skeleton generally has low strength relative tocarbon fibers, the matrix that is deposited imparts sufficientstructural integrity where the open porosity and isotropy of the carbonskeleton offer an excellent structure for wet frictional applications.The distribution of pores there through is substantially uniform andprovide an interlaced network of conduits through which hydraulic fluidcan flow. Further, the nature of this structure also enables extremelyhigh densification levels (e.g., up to 90%), while retaininginterconnected pores throughout the structure. The lack of strength inthe carbon skeleton is made up for by the pyrolytic carbon or otherdeposit which provides the foam with the structural reinforcement thatis needed for applications, such as wet friction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0016] The FIGURE is a partially-schematic cut-away drawing of a reactorsuitable for densifying a ceramic foam by the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] A description of preferred embodiments of the invention follows.The features and other details of the method of the invention will nowbe more particularly described with reference to the accompanyingdrawing and pointed out in the claims. It will be understood that theparticular embodiments of the invention are shown by way of illustrationand not as limitations of the invention. The principal features of thisinvention can be employed in various embodiments without departing fromthe scope of the invention.

[0018] In a method of this invention, a densified composite foam isformed by depositing a coating on an open-cell reticulated foamskeleton.

[0019] In one embodiment of the method, the foam skeleton comprisescarbon in the form of an open lattice of ligaments, wherein theinterconnected pores defined by the lattice have diameters of about 0.5to about 1.0 mm. The lattice has a micrographic porosity of about 100pores per inch, a bulk density of about 0.04 g/cm³, and a surface areaof about 1.6 m²/g. In an alternate embodiment, the lattice has amicrographic porosity of about 60 pores per inch. The foam skeleton isformed by pyrolizing a polymeric foam. The polymeric foam is formed froma thermosetting polymer foam preform of for example, a polyurethane,phenolic or polyimide. The polymer foam is in the form of an openlattice of ligaments forming a network of three-dimensionallyinterconnected cells. In comparison to carbon fibers, the polymer foamis relatively easy to shape into the desired form.

[0020] The polymer foam is liquid infiltrated with a carbon-bearingresin and pyrolized at approximately 600° to 1,200° C. in a vacuum or inan inert or reducing atmosphere to form an all-carbon reticulated foamskeleton. The carbon foam skeleton has an essentially amorphousstructure in the form of the polymer foam preform. As an alternative topyrolizing a polymer foam to form the carbon foam skeleton, the carbonfoam skeleton can be purchased commercially from Ultramet (Pacoima,Calif., U.S.A.) or from Vitre-Cell, Inc. (Essexville, Mich., U.S.A.).

[0021] As an alternative to the amorphous carbon skeletons, describedabove, a crystalline foam skeleton can also be used. For example, amicrocellular graphitic foam is formed by pyrolysis and graphitizationof thermoplastic carbon-fiber precursor materials, such aspolyacrylonitrile (PAN) or mesophase pitch. The foaming process alignsthe graphitic planes along each ligament axis, resulting in higherstrength and stiffness. These foams may be available from, for example,Wright Materials Research Co. (Dayton, Ohio, U.S.A.).

[0022] The carbon skeleton is then coated, in this embodiment, withpyrolytic carbon or graphite to increase the density of the foam and toincrease its strength to thereby make it suitable for use, for example,in wet friction applications. In one specific method, the carbonskeleton is coated with pyrolytic carbon via a process known as “rapiddensification.” In this example, the carbon skeleton is immersed in aliquid precursor and heated. The heat of the carbon skeleton pyrolizesthe liquid precursor, thereby generating gaseous products which depositcarbon on the skeleton upon contact to form a coating. The specifics ofthe rapid densification process and the equipment used to perform itwill now be described in greater detail and with reference to theFIGURE, below.

[0023] An example of apparatus suitable for practicing the method of theinvention is shown in the FIGURE. General descriptions of operation ofthe apparatus are set forth in U.S. Pat. No. 5,389,152, issued toThurston et al. on Feb. 14, 1995, the teachings of which areincorporated herein by reference in their entirety, and in U.S. Pat. No.4,472,454, issued to Houdayer et al. on Sep. 18, 1984, the teachings ofwhich are also incorporated herein by reference in their entirety.Reactor 100 is described in U.S. Pat. No. 5,397,595, issued to Carrollet al. on Mar. 14, 1995 and in U.S. Pat. No. 5,547,717, issued toScaringella et al. on Aug. 20, 1996. The teachings of both areadditionally incorporated herein by reference in their entirety. When aninduction coil 104 is used to heat the skeleton, reactor 100 ispreferably made from non-magnetic materials, such as aluminum, quartz,glass, stainless steel, ceramic or combinations thereof.

[0024] As shown in the FIGURE, reactor 100 defines cavity 102 in whichone or more carbon foam skeletons (not shown) are densified to formcomposite foams in accordance with the methods of this invention. Inoperation, cavity 102 is filled with a liquid precursor sufficient to atleast cover the skeleton. The liquid precursor is a liquid whichvaporizes and decomposes within the skeleton to deposit a decompositionproduct of the precursor at a temperature to which the skeleton can beheated. Depending upon the composition of the liquid precursor, thedecomposition product can be carbon, silicon carbide, silicon nitride,or another material. The liquid precursor should also be a dielectric.Preferably, the dielectric constant of the liquid precursor should beabove 0.5, more preferably above one, and most preferably above 1.5. Todeposit carbon within the foam, a hydrocarbon with an appropriateboiling point, such as cyclohexane, n-hexane or benzene is used.Alternatively, methyltrichlorosilane, dimethyldichlorosilane,methyldichlorosilane or other organosilane or organosilane mixtures areused to deposit silicon carbide. Also, the liquid precursor can bechosen to co-deposit materials. For example, a mixture of siliconcarbide and silicon nitride can be deposited using tris-n-methyl aminosilane or other silane compound.

[0025] One or more induction coils 104 are positioned within cavity 102,and the carbon foam skeleton is submerged in the liquid precursor inclose proximity to induction coil 104. In a specific embodiment, thefoam skeleton is placed in a support fixture to firmly hold the skeletonat a fixed position in relation to reactor 100 and coil 104. The exactshape of the fixture is adapted to the shape of the skeleton. Such afixture can be supported in any convenient way, such as on lip 132.Further, the size and shape of coil 104 preferably conform to the sizeand shape of the carbon skeleton. Induction coil 104 can be formed fromcopper or other highly conductive material which does not react with theliquid precursor even if heated. In one embodiment, induction coil 104is a Litzwire coil. In contrast to a standard single-strand coil, aLitzwire coil has an appearance similar to a telephone cable; i.e., aLitzwire coil comprises a bundle of insulated wires wound together toprovide more efficient inductive coupling.

[0026] Induction coil 104 is connected to busses 106 at connector 134.Connector 134 provides a link in an electrical circuit defined, in part,by busses 106. It also provides a link in the water flow circuit formedby channels 105. In particular embodiments, connector 134 is a block ofmetal allowing anchoring points for screws (not shown) to hold the baseof induction coil 104 to busses 106. The joints in the water flowcircuit can be sealed by flexible “0” rings or in some other convenientfashion. The material should be resistant to degradation in both waterand the liquid precursor. Viton® fluoroelastomer from E. I. DuPont deNemours & Co. or silicone rubber can be used for this purpose. Otherattachment arrangements, such as slots and grooves or clips, can also beused.

[0027] Busses 106 supply electrical energy to induction coil 104 and aremade of a highly conductive material, such as copper. Currents on theorder of hundreds of amperes to thousands of amperes are preferably usedto provide sufficient power to heat the foam. Because of the largeamount of current, busses 106 should have sufficient cross sections toavoid excess heating. Busses 106 can contain water passages 105 to carrycooling water through busses 106 and through induction coil 104.

[0028] In an alternative embodiment, busses 106 are connected to aheated mandrel, rather than to induction coil 104. The mandrel is theninserted through a foam tube which is to be the subject of rapiddensification. The mandrel is formed principally of carbon and mayinclude a release agent, such as boron nitride on its surface. In oneembodiment, the mandrel is used as a susceptor and is inductively heatedto generate heat which is conducted into the carbon foam. Alternatively,the mandrel is physically coupled with a voltage source and is subjectto resistive heating to generate heat to be transferred to the carbonfoam. Indeed, the substrate can be heated directly by resistance heatingprovided that the geometry is conducive to uniform heating.

[0029] Busses 106 enter the chamber through a silicone rubber seal 107.Busses 106 are connected to an AC power supply (not shown). The voltage,current, frequency and shape of induction coil 104 are determined by theshape, geometry and the electrical properties of the foam using knowntechniques.

[0030] As will be explained more fully, below, the rapid densificationprocess, described herein, heats the foam to higher temperatures at itscenter than at its periphery. Typically, the initial power provided bythe AC power supply is at a level that inductively heats the foam togenerate a temperature at the center of the foam that is high enough topyrolize the precursor and deposit a decomposition product, withoutheating more peripheral regions beyond relatively-low temperatures atwhich deposition is limited. In contrast, if the power, and theconsequent rate of deposition, is too high, the interior of the foam canbe sealed off by the deposition and build-up of decomposition product inperipheral areas, thereby resulting in non-uniform densification.Accordingly, the current level, in specific embodiments of this method,is on the order of thousands of amperes, though the precise level willdepend on the foam's cross-sectional area. At the densification center,which is not necessarily, but is typically, at the center of the foam,the temperature is typically in the range of between about 850° andabout 2,000° C. The preferred temperature is in the range of betweenabout 850° and about 1,000° C.

[0031] The dynamics of deposition by way of the above-described methoddiffers significantly from that of chemical vapor deposition (CVD).Whereas CVD deposits a decomposition product throughout the preformduring densification, the above-described method results in adensification profile that typically begins at the center of the preformand progresses to the surfaces. It is believed that this profile existsbecause the liquid precursor, and the boiling thereof, acts to cool theexterior of the foam, thereby creating a temperature gradient throughthe foam's thickness. Accordingly, the temperature gradient is such thatthe densification center of the foam is hotter than the surface. Becausethe rate of deposition increases with increasing temperature,densification proceeds from the center to the surface as the temperatureof the foam is increased. Further, as deposition of the decompositionproduct proceeds, the conductivity of the foam increases, improving thecoupling with the electric field. Consequently, less current is neededto heat the foam, and the foam can be processed using a modified heatingcycle in which the final power is decreased by about twenty-five percentfrom the power required to densify out to the edges of the foam.

[0032] Returning to the FIGURE, the liquid precursor that is to bepyrolized and deposited within the foam is supplied to reactor 100through precursor input 108 via valve 110. Initially, chamber 102 isfilled with a liquid precursor of sufficient quantity to cover the foam.In operation, the liquid precursor can be consumed in the depositionreaction or can escape from reactor 100 as vapor. Accordingly, precursorinput 108 can be utilized during operation of reactor 100 to replaceliquid precursor which is dissipated.

[0033] During densification, the liquid precursor can become clouded. Inthat case, valve 114 is opened to allow liquid precursor to flow throughreactor 100 and return 112 to filter 116 where it is filtered and pumpedback into reactor 100. Filter 116 can be any suitable filter, such as aporous ceramic screen or, more preferably, charcoal. Alternatively, theliquid precursor is removed from reactor 100 and is then distilled afterone or more densification cycles once the liquid precursor becomesclouded.

[0034] The liquid precursors, as used herein, are potentially flammable.Accordingly, the densification operation is preferably performed in aninert atmosphere. For example, nitrogen gas can be used. To purgechamber 102 of air, valve 120 is opened to allow an inert gas, such asnitrogen, to flow through input 118. Valve 124 can be opened to morerapidly and effectively purge vapor recovery system 130. Once theatmosphere in chamber 102 is replaced by an inert gas, such as nitrogengas, valve 128 can be opened to provide nitrogen directly into ventstack 136. This flow of nitrogen prevents air from reaching chamber 102and valves 120 and 124 can be closed. Closing valves 120 and 124 reducesthe flow of gas through vapor recovery system 130. Vapor recovery system130 can therefore operate more efficiently.

[0035] Vapor recovery system 130 is a system of the type known in theart for recovering vaporized liquids. Such a system reduces the amountof waste generated in the process and the amount of precursor used.Further, vapor recovery system 130 prevents the loss of a significantvolume of the liquid precursor due to vaporization.

[0036] Specific embodiments of the method of this invention include theadditional step of performing CVD prior to rapid densification topreliminarily increase the density of the foam and to increase theinitial rate of densification during the rapid densification process.

[0037] In another optional step, CVD can be performed after a compositefoam has been densified by the above-described methods. In somecircumstances, the above-described methods can produce a foam that has agreater density at its interior than at its periphery. In thesesituations, CVD can then be used to complete densification and toprovide the foam with a more uniform density by preferentially buildingthe deposited coating near the surface of the foam.

[0038] In regard to the product of this invention, a carbon foam of thisinvention includes an open lattice of carbon ligaments forming a networkof three-dimensional interconnected cells. The open lattice is coatedwith a crystallized carbon coating to produce a foam with a soliddensity greater than 30%.

[0039] In specific embodiments, the carbon/carbon foam composite of thisinvention is formed by the above-disclosed methods. Its structure isessentially isotropic, at least on a macroscopic scale. When designedfor wet friction applications, the composite structure preferably has asolid density of at least about 40%. Further, for some applications, thecomposite structure is subjected to rapid densification until a soliddensity of at least about 50% is obtained. Further still, with extendeddensification, openly-porous foam structures of 67%, 76% and 82% densityhave been obtained. The porosity can be controlled by varying the poresize and/or distribution of the polymer foam preform or by varying thedeposition conditions. In specific embodiments, the composite foamdefines a structure of interconnecting pores that allow hydraulic fluidto flow through the foam.

[0040] Accordingly, the foam can be cast in an appropriate shape andsufficiently densified to form a composite for use as a wet-frictionmaterial in a clutch or brake, e.g., in an automobile, where thecomposites are mounted opposite one another for relative rotation andfrictional engagement.

[0041] Moreover, fully or nearly-fully dense structures can be formedfor use in structural applications, such as aircraft brakes, automobilepistons, missile nose cones, susceptor/crucible cradles for crystalpulling, and low-coefficient-of-thermal-expansion space structures.

[0042] In additional embodiments, the composite foam is subjected topost-densification treatments, including surface coatings for improvedoxidation resistance, resin infiltration, or localized hardening byfilling (e.g., by painting or wicking) pores with various matrixmaterials. The surfaces of partially-densified foam can also be sealedover by spiking the temperatures at the end of rapid densificationprocessing to yield stiff, lightweight, porous structures having solidsurfaces.

[0043] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A carbon-carbon composite foam, comprising: a) anopen lattice of carbon ligaments forming a network ofthree-dimensionally interconnected cells; and b) a pyrolytic carboncoating on the open lattice of carbon ligaments, wherein thecarbon-carbon composite foam has a solid density of greater than 30%. 2.The carbon-carbon composite foam of claim 1, wherein the carbon-carboncomposite foam defines a structure of interconnecting pores that allowhydraulic fluid to flow through the carbon-carbon composite foam.
 3. Thecarbon-carbon composite foam of claim 2, wherein the carbon-carboncomposite foam has an essentially-isotropic structure.
 4. Thecarbon-carbon composite foam of claim 1, wherein the open lattice ofcarbon ligaments consists essentially of amorphous carbon.
 5. Thecarbon-carbon composite foam of claim 1, wherein the carbon-carboncomposite foam has a solid density of at least about 40%.
 6. Thecarbon-carbon composite foam of claim 1, wherein the carbon-carboncomposite foam has a solid density of at least about 50%.
 7. Thecarbon-carbon composite foam of claim 1, wherein the foam includes athermosetting polymer selected from the group consisting ofpolyurethanes, phenolics, and polyimides.
 8. The carbon-carbon compositefoam of claim 1, wherein coating is included between a reticulated foamskeleton and a by-product of a liquid precursor.
 9. The carbon-carboncomposite foam of claim 8, wherein the deposited coating includescarbon.
 10. The carbon-carbon composite foam of claim 1, wherein acoating is formed on a by-product of a liquid precursor.
 11. Thecarbon-carbon composite foam of claim 1, wherein the product of apyrolyzed liquid precursor is any one of carbon, silicon carbide, andsilicon nitride.
 12. The carbon-carbon composite foam of claim 1,wherein a liquid precursor for forming the product is a dielectric. 13.The carbon-carbon composite foam of claim 14, wherein the dielectricconstant of the liquid precursor is at least 0.5.
 14. The carbon-carboncomposite foam of claim 1, wherein a liquid precursor for forming theproduct is selected from a group consisting of cyclohexane, n-hexane,benzene, methyltrichlorosilane, dimethyldichlorosilane,methyldichlorosilane, and tris-n-methyl amino silane.
 15. Thecarbon-carbon composite of claim 1, wherein a chemical vapor depositioncoating is deposited on the pyrolytic carbon coating.
 16. Thecarbon-carbon foam of claim 1, wherein said foam includes pores being inthe range of between about 500 and about 1,000 microns in diameter. 17.The carbon-carbon foam of claim 1, wherein said foam includesmicrographic porosity in the range of between about 60 and about 100pores/inch.
 18. The carbon-carbon foam of claim 1, wherein said foamincludes a bulk density of about 0.04 g/cm³.
 19. The carbon-carbon foamof claim 1, wherein said foam includes a surface area of about 1.6 m²/g.